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Thursday, 18 January, 2001, 17:39 GMT

Light stopped in its tracks

By BBC News Online science editor Dr David Whitehouse

Scientists say they have slowed light to a dead stop, stored it and then let it go again.

Normally light is the fastest thing there is, travelling at about 299,000 kilometres a second (186,000 miles a second). But it is slowed slightly when it moves through some materials, such as glass.

The researchers have taken this effect to the extreme and say they have effectively made a beam of light stop after it entered a specially designed gas chamber.

The experiment has been hailed as a landmark that could pave the way for faster computers and totally secure communications.

All stop

The breakthrough has been achieved by two independent teams of researchers. One was led by Dr Lene Vestergaard Hau, of Harvard University, and the other by Dr Ronald Walsworth, of the Harvard-Smithsonian Center for Astrophysics, both in Cambridge, Massachusetts, US.

Dr Lene Hau has worked to make light travel slower and slower

Two years ago, Dr Hau surprised the world when she slowed light to about 60 kph (38 mph) by passing it through chilled sodium gas. And then last year, she reported slowing light down to 1.6 kph (one mile an hour) - slower than a slow walk.

Transparent media like water or glass can slow a light beam slightly. The effect causes the phenomenon of refraction and is the basis for lenses and prisms.

To stop light altogether, the scientists have utilised a similar but far more powerful effect. The researchers cooled a gas of magnetically trapped sodium atoms to within a few millionths of a degree of absolute zero (-273 deg C).

Huge impact

This would normally be opaque to light. But by illuminating it with a laser called a coupling beam, it can be made transparent, thereby allowing another laser pulse to pass through it. It is a process known as electromagnetically induced transparency.

And, astonishingly, if the coupling laser is turned off while the probe pulse is inside the gas cloud, the probe pulse stops dead in its tracks. If the coupling beam is then turned back on, the probe pulse emerges intact, just as if it had been waiting to resume its journey

The biggest impact of this work could be in the burgeoning field of quantum computing and quantum communication.

In theory, quantum computers, in which information is stored in the quantum states of atoms, could be very much faster than existing machines. And quantum communications could never be eavesdropped.

The research is to be published in forthcoming issues of the journal Nature and the Physical Review Letters.